1. Field of the Invention
The present invention relates to a soft polyurethane foam suitably used as a material of interior members of automotive vehicles, and a method of production thereof. In particular, the invention is concerned with such a soft polyurethane foam used in the vehicle interior members such as an energy absorbing headrest capable of protecting a passenger of the vehicle from suffering from a whiplash injury, and a seat back of a vehicle seat exhibiting good energy absorbing properties. Further, the invention is concerned with methods of producing the soft polyurethane foam.
2. Discussion of the Related Art
A soft polyurethane foam has been widely used in vehicle interior members such as a seat cushion, a headrest, an instrument panel and a steering wheel, as well as furniture and bedding, taking advantage of its high rebound resilience property. The soft polyurethane foam is required to exhibit desired properties depending upon its applications. In particular, when the soft polyurethane foam is used in the vehicle headrest, it is required to exhibit high rebound resilience property and suitable hardness, so as to give a good feel to the vehicle passenger, in other words, good cushioning characteristics.
In recent years, the vehicle is equipped with various kinds of energy absorbing devices in an attempt to protect a passenger of the vehicle upon a vehicle collision accident. For instance, an air bag is installed in the steering wheel, instrument panel or doors. As disclosed in JP-A-8-193118, a rigid polyurethane foam is provided in the inside of the door, or in the side portion of the seat back adjacent to the door, for the purpose of absorbing the energy applied to the vehicle upon its collision, to thereby protect the loins, shoulders and abdomen of the passenger upon the collision.
In general, the rigid polyurethane foam does not restore its original shape once it is deformed by stresses applied thereto. Accordingly, the rigid polyurethane foam is used in limited portions of the vehicle. That is, the rigid polyurethane foam cannot be used as a material of the vehicle interior members such as the headrest, seat back, instrument panel, steering wheel, door trim and pillar, which members are frequently contacted by the vehicle passenger and subject to stresses due to the frequent contact.
JP-A-2-52607 and JP-A-1-280413 disclose a seat cushion having an improved comfort as felt by the passenger. The disclosed seat cushion is formed of a combination of a soft polyurethane foam of low rebound resilience whose percentage is not higher than 25% and a soft polyurethane foam of high rebound resilience whose percentage is not lower than 55%. JP-A-7-67757 discloses a pillow which uses as a core material a soft polyurethane foam having a rebound resilience percentage of not higher than 5%, to thereby keep the head of the user in a stable state while the user is sleeping. JP-A-2-175713 discloses a method of producing a soft polyurethane foam of low rebound resilience whose percentage is not higher than 25% by using a polyether polyol having an average molecular weight of 400-2000 and an average functionality of 2-3.5.
The vehicle interior members which use the soft polyurethane foam having the low rebound resilience property assure good energy absorbing characteristics as well as a good feel as given to the passengers. Since the soft polyurethane foam with the low rebound resilience property can restore its original shape after it has been subjected to compression deformation, the soft polyurethane foam can be suitably used in the interior members for the energy absorbing purpose, unlike the rigid polyurethane foam described above.
In the conventionally proposed soft polyurethane foam having the low rebound resilience property, however, the hardness of the resin which gives the soft polyurethane foam rapidly increases with a decrease in the temperature of the soft polyurethane foam. In this case, the soft polyurethane foam is too hard to exhibit its excellent properties. For instance, when the soft polyurethane foam with the low rebound resilience property is used as a pad material for the vehicle headrest, it is soft enough to assure a good feel at the room temperature of 10.about.30.degree. C. In a cold condition at the temperature of 0.about.-20.degree. C., however, the resin hardness of the soft polyurethane foam undesirably rises to a value as high as that of the rigid polyurethane foam, deteriorating the energy absorbing characteristics of the soft polyurethane foam. In this case, the soft polyurethane foam is not likely to restore its original shape immediately after it has been subjected to compression deformation, and the feel as given to the vehicle passenger is considerably deteriorated.
In an attempt to lower the rebound resilience percentage of the soft polyurethane foam for improving its energy absorbing characteristics, the soft polyurethane foam used in the vehicle interior member is formed by using a polyol material whose average molecular weight is relatively low. However, when the polyol material with the low average molecular weight is used in forming the soft polyurethane foam, it undesirably takes a lot of time for the reaction with a polyisocyanate component for forming the polyurethane. In this case, when the soft polyurethane foam is obtained by a foaming operation using a suitable mold, it requires a relatively long period of time before the formed polyurethane foam can be removed from the mold. In other words, it requires a relatively long period of time to completely cure the polyurethane foam, so that its production efficiency is deteriorated. Thus, the use of the polyol material with the low average molecular weight is not practically desirable.
It is well known to use a mono-ol or a monohydric alcohol in producing the polyurethane foam. For instance, JP-A-52-21097, JP-A-2-163112, JP-A-3-109412, JP-A-4-146916, JP-A-5-202161 and JP-A-5-287047 teach the use of the monohydric alcohol in the reaction system for forming the soft polyurethane foam, whereby the obtained soft polyurethane foam has a desired softness value and a reduced weight, without suffering from scorching. JP-A-60-177035, JP-A-3-185018 and JP-A-7-18045 disclose that the use of the monohydric alcohol prevents an increase of the viscosity of materials used for producing the polyurethane, so that the materials can be sufficiently blended. JP-A-56-36514, JP-A-60-219220, JP-A-62-238256, JP-A-2-199111 and JP-A-5-84858 disclose the use of the monohydric alcohol in an attempt to prevent shrinkage of the obtained polyurethane foam upon its removal from the mold, and improve its mechanical strength, storage stability and soundproof characteristics in a lower frequency range. None of those publications, however, teach a soft polyurethane foam which exhibits low rebound resilience characteristics both at the room temperature and under the cold condition while assuring an excellent feel as given to the user. Moreover, none of the publications suggest the use of such a soft polyurethane foam as an energy absorbing member.